The present invention relates to steering shafts for impact-energy absorbing steering columns and a manufacturing method thereof. More particularly, the present invention relates to an improvement in engagements such as serration for providing steering shafts that are contractible for impact-energy absorption.
JP-A 948353 shows a steering shaft comprising a hollow lower shaft and a hollow upper shaft integrated with each other. A circumferential groove is formed in the outer peripheral surface of the lower shaft, whereas a hole is formed in the upper shaft. The groove and the hole are filled with a resin.
In the event of vehicle collision, a secondary collision will occur which a driver collides with a steering wheel by law of inertia. This produces a great impact load applied axially to the upper shaft. Thus, the upper shaft is moved downward with respect to the lower shaft, providing a shearing force to the resin. When a load applied to the upper shaft exceeds a maximum shearing stress for the resin, the resin is sheared to allow movement of the upper shaft toward the lower shaft. This movement contributes to absorption of an impact of the upper shaft applied to the driver.
However, the above steering shaft needs machining of the lower and upper shafts and injection of the resin, leading to high manufacturing cost. Moreover, variations between products often occur due to individual difference, which causes increased number of processes for control of the resin-injection pressure, clamping pressure, etc., resulting in high manufacturing cost.